The invention relates to the determination of a suitable pressure to test for correct mask fitting, and for correct mask fit.
Sleep disordered breathing, such as Obstructive Sleep Apnea (OSA), is treated with devices which provide positive pressure ventilatory assistance, such as Continuous Positive Airway Pressure (CPAP) devices. A typical device comprises a controllable flow generator coupled to a nasal mask that provides a supply of breathable gas to a patient in the range 4 to 30 cm H2O positive pressure. Furthermore, a field of ventilatory assistance known as noninvasive positive pressure ventilation (NIPPV) supplies a patient with various pressures at appropriate times during the patient""s breathing cycle. Throughout this specification a reference to xe2x80x9cCPAPxe2x80x9d or xe2x80x9cventilatory assistance apparatusxe2x80x9d is to be understood as including a reference to CPAP or non-invasive positive pressure ventilation. Nasal prongs, a mouth mask or fill face mask may be used as alternatives to a nasal mask. A reference to a mask herein is intended to include a reference to any of these patient interface devices.
Apparatus, and thus treatment efficacy depends on correct mask fitting to reduce or eliminate leaks. A known arrangement of CPAP apparatus (for example) provides a test mode, which may be used prior to the functional (or operational) mode, whereby be user can test-fit the mask. Whilst in this test mode, the apparatus provides a test pressure. This test pressure may be, for example, fixed at 10 cm H2O or alternatively the maximum output pressure of the device. During this test mode the mask is fitted to the patient so as to avoid leaks that might occur at the test pressure.
Another known arrangement is that the mask test pressure is chosen to be a function of the minimum and maximum pressure settings. For example:
Mask-fit test pressure=Pminxe2x80x94set+0.75(Pmaxxe2x80x94setxe2x88x92Pminxe2x80x94set)xe2x80x83xe2x80x83(1)
where
xe2x80x83Pminxe2x80x94set=Minimum pressure setting
Pmaxxe2x80x94set=Maximum pressure setting
The problem with these known methods is that the test pressure will be independent of the pressures actually delivered during an automatically titrating mode. During the automatically titrating mode (or autotreatment mode) the device varies the pressure delivered in the mask in accordance with the patient""s requirements while the patient sleeps. Examples of devices and methods of treatment that operate in such an automatic mode can be found in commonly owned U.S. Pat. No. 5,704,345 (Berthon-Jones assigned to ResMed Limited) and WO 98/12965 (Berthon-Jones assigned to ResMed Limited).
A good indication of mask fitting under normal conditions of use will not be obtained if the test pressure is significantly different to the pressures encountered in normal use. No leaks may be detected during the test mode, but during the functional mode, the mask may leak. Alternatively, the mask-fit test pressure may be unnecessarily high and discourage the patient from using the mask-fit feature, or from using the device due to the discomfort resulting from the test pressure suggesting that the patient fit the mask with a strap tension that is greater than would be necessary in practice.
FIG. 1 shows a cumulative frequency plot for overnight treatment using an automatically titrating CPAP device for two patients, (Axe2x80x98_xe2x80x99,Bxe2x80x98-.-.-xe2x80x99), together with the 95th percentile (- - - - -). The sampling rate was 1 per minute. For patient A, there were 375 pressure readings and the 95th percentile pressure was approximately 7 cmH2O. For patient B, there were 79 pressure readings and the 95th percentile pressure was approximately 20 cm H2O. If the prior art solution using equation (1) had been adopted, with the 4 cm and 20 cm minimum and maximum pressure settings respectively, the mask-fit pressure would have been 16 cm H2O. This pressure is much higher than necessary for patient A, and may have been insufficient for patient B.
It is an object of the invention to overcome or at least ameliorate these problems, achieved by providing for the adaptive determination of the mask-fit test pressure based on prior use.
The invention discloses a method for determining a mask-fit test pressure to be applied to a wearer""s mask by ventilatory assistance apparatus, the method comprising the step of:
determining a percentile pressure of a previous ventilatory assistance session to be said test pressure.
The invention further discloses a method for assessing correct fitting of a mask delivering ventilatory assistance, provided by ventilatory assistance apparatus, to a wearer of the mask, the method comprising the steps of:
determining a percentile pressure of a previous ventilatory assistance session to be applied as a test pressure;
determining leak flow from said mask at the test pressure; and
displaying or otherwise indicating the magnitude of the leak flow as an indication of correct mask fitting.
The invention further discloses ventilatory assistance apparatus comprising:
a controllable flow generator providing a positive pressure of breathable gas;
a conduit coupled to the flow generator to receive said gas;
a mask to be worn by a wearer, in turn, to receive said gas from said conduit at a desired pressure; and
a controller having control of said flow generator, and operable to cause a mask-fit test pressure to be applied at the mask, said test pressure being determined as a percentile pressure of a previous ventilatory assistance session.
The invention yet further discloses ventilatory assistance apparatus as defined above, further comprising:
flow sensor means, for sensing respiratory flow, passing a flow signal to the controller; and
display or indication means; and
wherein the controller is further operable to determine mask leak flow at the test pressure from the respiratory flow signal, and to cause the display or indication means to display or otherwise indicate the magnitude of the leak flow as an indication of correct mask fitting.
It is preferred that the indication of correct mask fitting is quantised as a degree of leak. The mask leak flow can be compared against a threshold value, representing the no leak degree, to determine whether there is correct mask fitting if the threshold value is not exceeded.
Advantageously, if there has been no previous session the test pressure is chosen to be a base pressure. The percentile pressure can be in a range between the 75th and 95th percentile pressure. Further, the base pressure can be in the range 10-12 cm H2O.
In a preferred form, the ventilatory assistance apparatus can have an automatic pressure mode in which case the respective steps recited above for controller operation are performed, and a manual pressure mode in which the currently set ventilatory assistance pressure is chosen to be the test pressure.
The test pressure can be applied for a period of time, for example 3 minutes.